1. Field of the Invention
The present invention relates to apparatus for the manufacture of flat glass wherein the glass is formed while being supported on a surface of a pool of molten metal following delivery thereto as molten glass flowing from a glass melting, refining and conditioning furnace. More particularly, this invention relates to a combination of elements providing an improved means for delivering molten glass at a controlled rate from a glassmaking furnace and for initially establishing a body of molten glass on the surface of a pool of molten metal in a forming chamber.
2. Brief Description of the Prior Art
It is well known that molten glass can be delivered onto molten metal and formed into a continuous ribbon or sheet of glass. In accordance with the teachings of Pilkington U.S. Pat. No. 3,220,816, it is disclosed that a suitable method for delivering molten glass onto a surface of molten metal for forming is to direct the flow of molten glass over a spout or lip extending over and above a pool of molten metal and to allow the molten glass to fall freely from the lip onto the surface of the pool of molten metal to form a body of molten glass thereon by spreading laterally and flowing both rearwardly and forwardly from the point at which it is poured onto the molten metal. In such a delivery system it is common to provide a refractory element known as a wetback block which is contacted by the rearwardly flowing portion of the delivered body of molten glass and which serves to divert and direct the rearwardly flowing portion of molten glass outwardly so that it may join the forwardly flowing portion of the body of molten glass along its marginal regions.
U.S. Pat. No. 3,508,902 illustrates a molten glass delivery system along with the inlet end of a glass forming chamber employing a wetback block for engaging a rearwardly flowing portion of a delivered stream or body of molten glass.
Molten glass delivery systems typically employ metering members which are used to control the rate of molten glass flow to the forming chambers associated with them. Typically, a metering member is a refractory gate or tweel which may be translatably adjusted in a molten glass flow channel. Such a metering member acts as a gate valve to control the flow of molten glass through the channel. As illustrated in U.S. Pat. No. 3,445,217, a metering member may be a composite tweel employing two different refractories. One refractory may be selected as a wear-resistant refractory as shown in this patent with the wear-resistant refractory serving as the lower glass-contacting portion of the tweel, and as illustrated in this patent, the upper portion of the tweel may comprise a refractory material selected for its resistance to thermal shock and its mechanical strength.
U.S. Pat. No. 3,445,217 suggests the use of prefired fire clay or prefired clay bonded calcined kyanite or sillimanite as the upper portion of a tweel and suggests that the lower portion of a tweel be a wear-resistant refractory material such as fused cast zirconia, alumina or corundum material.
From time to time in the operation of a glass forming facility, the glass produced develops slight defects which, while not readily apparent to the naked eye, are nevertheless undesirable in the glass. These defects include extremely fine bubbles or seeds in the bottom surface of the glass, particularly in its marginal portions which are attributed to contamination along the wetback region of such a facility. Defects also include fine lines in the top surface of glass, which lines have been attributed to the non-uniform wear of tweels. Such non-uniform wear has, in turn, been attributed to inhomogeneities in the refractory material from which glass-contacting portions of tweels have been fabricated. In accordance with the present invention, the glass-contact portion of a tweel and the wetback block of a glass forming facility are made of material which is less susceptible of causing such defects.